Brake choppers are commonly used in frequency converters for dissipating regenerated energy that cannot be fed back to the supplying network. For example in a case where a motor supplied by a frequency converter with a DC voltage intermediate circuit is rotated by the load, the motor acts as a generator and feeds power back to the supply. When the rectifying bridge of the frequency converter is not configured to feed the regenerated power back to the network, the voltage of the intermediate circuit starts to increase.
When the voltage of the intermediate circuit has increased to a limit which is higher than the nominal voltage of the intermediate circuit, the brake chopper, which is a circuit comprising a controllable switch and a resistor connected between the rails of the intermediate circuit, activates and starts to dissipate the energy in the resistor and thereby reduces the voltage. The brake chopper reduces the voltage until it is within acceptable limits from the nominal voltage.
The brake choppers operate based on measured voltage of the intermediate voltage circuit, independently of the other controls of the frequency converter. This means that when the voltage of the intermediate circuit rises above the set limit, the brake chopper starts its operation based on the voltage independent of other control operations carried out at the same time. The operation of the brake chopper may be based for example on pulse width modulation. According to the PWM operation principle a switching period is determined and the brake resistor is kept in operation inside the switching period for a time period that is dependent on the magnitude of the voltage. The simplest solution for the operation of a brake chopper is, however, to keep the resistance connected between the rails of the intermediate voltage until the voltage has decreased to an acceptable level.
Brake choppers are especially used in low-cost and smaller size frequency converters or in situations where the regenerated energy cannot be fed back to the supplying network. In low-cost and smaller size frequency converters the additional cost of a rectifier which could feed power back to the supplying network would increase the costs and make the converter less appealing. Further on small power levels the amount of power dissipated in the chopper resistor is relatively small.
FIGS. 1, 2 and 3 show typical semiconductor modules for use in a power device, such as a frequency converter. These kinds of internally connected modules are cost effective building blocks that are commonly used in frequency converters. The module in each of the FIGS. 1, 2 and 3 comprises an inverter bridge 2 and a brake chopper 1. The module comprises phase outputs A, B, C, inputs for intermediate circuit UDC+, UDC− and a connection point for brake resistance BRK. The brake resistance is connected to the module between the positive intermediate circuit connection point UDC+ and the connection point BRK.
In FIG. 1 the inverter part 2 and the brake chopper 1 are not connected internally to each other. Thus one can add electrical components between these parts of the module. Components that may be inserted between the inverter and brake chopper include, for example, capacitors and measurement devices. In the module of FIG. 2 the inverter part 2 and the brake chopper 1 are connected together internally in the positive rail UDC+ of the intermediate circuit i.e. the positive end of the brake chopper is connected to the inverter part and the negative end is left unconnected. In the module of FIG. 3 the inverter 2 and the brake chopper 1 are internally connected together, giving no possibility of inserting electrical components between the inverter and the brake chopper.
Another feature in frequency converters that is often used for lowering the costs is the determination of motor phase currents by using only one current sensing device. This device is arranged to measure DC current in the intermediate circuit. Although only one measurement is obtained, the measured value can be allocated to a specific motor phase once the inverter switch combination during the time of the measurement is known. In FIGS. 1 and 2 this kind of measurement device can be connected between the brake chopper and the inverter, whereas in case of the module of FIG. 3, the measurement device has to be connected to the DC side of the brake chopper.
It has been noticed that the structure of FIG. 3 with DC current measurement from the DC intermediate circuit is problematic. This is due to the fact that current flowing through the brake chopper sums to the motor current and the current sensing device senses the summed current. If the current detection device is arranged in the structure of FIG. 3, the detected current is different than the phase current. This is further illustrated in FIGS. 4 and 5.
FIGS. 4 and 5 show the module of FIG. 3 connected to a motor and having current measurement arranged to the intermediate circuit. In FIG. 3 the controllable switch 5 of the brake chopper is in an open state and current cannot flow via the brake resistor R. Since the state of the inverter switches is 100, i.e. phase A is connected to positive intermediate circuit voltage UDC+ and phases B and C are connected to negative intermediate circuit voltage UDC−, the current measurement device 3 detects current IDC, which corresponds to the current of output phase A.
FIG. 5 shows the same output switch combination as in FIG. 4. Now the switch 5 of the brake chopper is conducting, and certain amount of current flows via the brake resistor R. The braking current, i.e. the current via the brake resistor, is shown in dashed line, and it can be seen that it closes via the current measurement device 3, which is arranged on the DC side of the brake chopper i.e. between the DC voltage source and the brake chopper. This current is summed with the motor phase current and the measurement does not give the desired value of phase current.
One possible way of dealing with the above problem is to calculate the magnitude of the current in the brake resistor and to subtract the calculated value from the measured one. The outcome of this subtraction would then be the desired current of the inverter bridge. This kind of approach requires information on the resistance of the used brake resistor, and brake chopper state information i.e. whether the chopper is conducting or not at the time of the current sample. The resistance can be identified during start-up of the converter, but since the resistance is dependent on the temperature, the identified value would not necessarily be valid during operation. This would directly distort the calculated result and render it useless. Further the voltage information needed in calculation of current according to Ohm's law should be measured accurately and simultaneously with the DC current for the calculated current to be accurate.
Another problem relating to DC current measurement and subtraction of braking current from the measured DC current is the saturation of the current measurement device. The sum of the inverter and brake resistor current can be so high that the current measurement device is saturated. DC current can be determined using a shunt resistor, and the measurement result is provided from the shunt resistor using an operational amplifier. Since the output of the operational amplifier cannot be higher than its positive operational voltage, a risk of saturation is evident. In this kind of situation the subtraction of braking current would be carried out from already distorted measurement, and the end result would also be erroneous.